a) Field of the invention
Industrial goods are typically cleaned and degreased during and after manufacturing, in preparation for the application of further surface treatment processes, in connection with their repair or maintenance during their lifetime or to preserve their appearance and functionality. The present invention relates to an improved method for cleaning and degreasing industrial goods including elimination of used degreasing solutions and impurities.
b) Background art
Cleaning and degreasing systems and methods used for such purposes are known. Earlier degreasing and cleaning systems often involve organic solvents. These solvents can be reused after separation from the impurities through redistillation. However, solvents pose an environmental problem because their vapor emissions are hazardous and the disposal of the impurities is complicated by their contamination with such solvents.
Cleaning and degreasing may be accomplished with less environmental impact through aqueous solutions, alkaline or acidic, which include tensides. The oils and greases are removed from technical products by emulsification or saponification with the tensides. Emulsification is separating the oil from the surface and breaking it up into microscopic droplets which become suspended in the solution. Saponification is dissolving the oils in the solution. The first produces an emulsion, the second a solution. To the extent that emulsified oils and undissolved solids are suspended in the degreasing solution it is not a true solution but a suspension. Nevertheless the term degreasing solution will be used generically, throughout this application, and this term will include both solutions and suspensions. Such aqueous solutions pose no problem with regard to harmful vapor emissions. However the separation of organic matter from the solution and the disposal of the organic matter and used solution do present severe practical problems due to the large volume of waste material which must be disposed of. In addition part of the degreasing solution is moreover carried out by the cleaned goods into subsequent treatment steps, which degrades their effectiveness.
Different methods have been proposed and are used to remove the organic content, especially the oils from such degreasing solutions.
One method is to skim floating oil drops off the surface of the solution. Oil floats in drops when the emulsifying capacity of the solution is exhausted due to saturation. This may also be purposely promoted by using a degreasing product with deliberately limited dispersing capacity for the emulsified oils. However this tradeoff limits the capacity of the degreasing product to separate the oil from the surface of the goods and to break it down for emulsification. In any case, skimming removes only part of the oil and it does not prevent the transport of degreasing solution into rinse baths and processes beyond. Neither oil nor tensides are eliminated.
Another method is to remove the emulsified oil by mechanical separation through filtration or centrifugation. This method can remove only emulsified oil, i.e. oil in suspension, while it cannot remove saponified oils, i.e. oils in solution. Therefore, mechanical separation removes only part of the oil and it does not prevent a transport of degreasing solutions into rinse baths and processes beyond. Neither oil nor tensides are eliminated.
Another method is to remove oils from the degreasing solution by chemical separation. The degreasing solution is demulsified by adding an appropriate agent e.g., another tenside. Tensides and impurities float to the surface and are skimmed off. New tensides are added to the solution to renew its emulsifying capacity and the refreshed solution is used again. This method does not prevent the transport of degreasing solution into rinse baths and processes beyond. Neither oil nor tensides are eliminated.
Still another method is to clean the rinse water by filtration through an activated carbon filter. This method is effective in removing organic materials, tensides and impurities, but it is useful only as a final cleaning stage. Without prior removal of the bulk of the organic material the carbon filter will immediately be overloaded and plugged. This method can be successful in preventing the transport of degreasing solution beyond the rinse stage but the procedure involved is expensive both in equipment and operating costs and oils and tensides are only separated and not eliminated.
Biological methods are a newer approach. EP 0 309 432 to Hakansson describes that a degreasing bath can be operated while maintaining a biological activity in the bath which eliminates the organic impurities by biological degradation. Bacteria mineralize the oils and greases. The best conditions are carefully chosen to control the bioactivity in such a way that only the impurities are eliminated. Elimination of the tensides is carefully avoided in order to preserve the degreasing bath. The degreasing bath is maintained at a pH of about 9 and at a temperature between 35.degree. to 45.degree. C. Hakansson also shows that a degreasing bath with higher pH and temperature may precede the biologically active bath in order to provide stronger degreasing action. In that case the second bath serves for the biodegradation of the impurities as above. The goods removed from either of the above mentioned baths are however contaminated with the degreasing solution which comprises organic matter such as oil, tensides and other impurities.
Though it has been proposed to spray water over the goods degreased according to any of the above methods in order to rinse off cleaning or degreasing liquid, it is not an applicable or appropriate solution because a large volume of rinsing liquid results which is contaminated with organic matter. This increases the amount of waste to be disposed of.
It is furthermore known to use closed-cycle-rinsing systems. However, when the rinse baths are closed-cycle-systems organic matter, tensides and further impurities accumulate in the rinsing solution, finally leading to a rinse solution which is too contaminated for further use. The rinse solution has then to be exchanged, and the used solution must be treated at a different site, again increasing the amount of waste to be treated.
But even before the rinsing solution is too contaminated for further use, organic matter, tensides and further impurities are present in the solution in continuously rising levels. These contaminants are carried over by the goods into further treatment processes leading eventually to goods of lower quality.
In the international application, published under the international publication No. WO 92/16314 Hakansson describes the use of his bioprocess in a device for cleaning objects. The process uses an optional rinse stage after cleaning. However, the rinse water only dilutes the degreasing solution; it also gets transported into subsequent treatment baths and the environment beyond. This bioprocess is intended to eliminate organic impurities in connection with degreasing and cleaning but it does not prevent the transport of degreasing solution into processes beyond; tensides are not eliminated.